In the field of medial diagnostics, assay-based sensor devices are rapidly gaining popularity because of the prospect of being able to accurately determine the presence and concentration of a wide variety of analytes in samples of interest. To this end, the analyte is attached to a detectable label such a fluorescent or chemoluminescent probe, an enzyme for converting a calorimetric substrate or a magnetic particle. Either the analyte or a further entity attached to the detectable label forms a selective bond, i.e. a highly specific binding with e.g. an antibody attached to a sensor area of the sensor device such that the concentration of the analyte can be detected from the presence of the detectable label in the sensor area upon the formation of the highly specific binding, for instance by exposing a fluorescent probe to electromagnetic radiation having a suitable wavelength for electronically exciting the probe or by exposing a chemoluminescent probe to an appropriate catalyst (e.g. heat) to initiate the chemoluminescent reaction and measuring the intensity of the emitted light in both instances or by placing the magnetic particles in an (electro)magnetic field and measuring the interaction of the particles with this field. For instance, the particles may be placed in a light beam where the amount of scattering of the light beam caused by the interaction with the magnetic particles can be quantified by measuring the intensity of the reflected light beam.
Many suitable specific binding pair candidates such as strong binding couples are known, which are typically based on a lock-and-key type interaction between a receptor molecule and a molecule, e.g. a drug. This makes assay-based sensor devices particularly suitable to determine the presence or absence of specific proteins and other biological compounds such as DNA, RNA, hormones, metabolites, drugs and so on, or to determine the activity and function of active and catalytic biomolecules such as proteins, peptides, prions, enzymes, aptamers, ribozymes and deoxyribozymes. For instance, immunoassays are already used to determine the specific amount of specific proteins in body fluids to aid further diagnosis and treatment.
Several different types of assays exist. An example of an immunoassay is the enzyme-linked immunosorbent assay (ELISA), in which two antibodies are used that bind two separate, non-overlapping epitopes on the antigen. This may be accomplished by using two monoclonal antibodies that selective bind to different discrete sites or by using affinity-purified polyclonal antibodies that have been raised to different epitopes on the antigen. One of the antibodies is bound to the sensor surface whereas the other antibody is labeled with an enzyme. The antigen concentration can be determined following a binding reaction and the rinsing of the binding site to wash away unbound material by measuring the amount of colorimetric substrate converted by the enzyme label attached to the second antibody. Due to the requirement of two antibodies, such assays are commonly referred to sandwich assays because the analyte of interest is sandwiched between these two different antibodies.
The competitive assay is another example of an immunoassay. Here, an epitope of the target molecule competes with a homologue expected to have a similar epitope to bind to a paratope on either a detectable label or a sensor surface. In the first case, the homologue is attached to the sensor surface. In the latter case, the homologue is attached to the detectable label. The concentration of the target molecule is determined indirectly from the detected label concentration on the binding site. Such assays are particularly useful for the detection of target molecules having a single epitope only, such that these target molecules cannot be detected using sandwich assays. Further examples of known assays can for instance be found in WO 2007/060601, and other examples will be apparent to the skilled person.
Assay-based sensors provide promising new opportunities in the field of medical diagnostics. Sensor devices utilizing the presence of a magnetic label on a biomolecule are of particular interest, for instance because the application of a magnetic field can accelerate the formation of the highly specific binding on the measurement site such that the binding reaction can be completed in a short time span, which opens up the possibility of offering assay-based sensor devices for diagnostic purposes to untrained staff, non-technical personnel, directly to the patient, and so on without the need for the presence of a medical professional.
As is the case with any type of assay-based device, misuse of the device can produce unreliable diagnostic results. For instance, the biomolecules that are attached to the sensor surface as the first part of the binding couple can degrade when the sensor device is exposed to adverse environmental conditions or has been otherwise damaged. Also, the correct procedure for preparing the sample and/or applying the sample may not be followed, e.g. the sample is incorrectly prepared, the wrong amount of sample is added and so on, which also can cause erroneous measurement signals.
Regulatory requirements dictate that for the distribution of some medical devices in such application domains, fail-safe mechanisms are present in the device for instance to indicate to a user that the device has become unreliable. For instance, CLIA waived regulatory approval is preferred for tests to diagnose e.g. heart attacks because this allows untrained operators of the medical device to perform the test. This approval is conditional on the presence of such fail-safe mechanisms. The presence of such fail-safe mechanisms is equally of crucial importance in other emergency situations where the sensor device is used for making clinical decisions. Hence, there exists a need to be able to predict the accuracy of a diagnostic result obtained with an assay-based sensor device.